1. Field of the Invention
The present invention relates to a method for searching an access channel and allocating a finger for a base station, and in particular to an improved method for searching an access channel and allocating a finger for a base station which is capable of implementing a mobile communication system such as a DCS(Digital Control System) using a CDMA(Code division Multiple Access), a PCS(Personal Communication System) and a satellite communication system.
2. Description of the Background Art
Generally, a mobile communication system is a communication system capable of receiving a communication service.
In the above-described mobile communication system, a mobile station is capable of transmitting a signal to a base station for receiving a mobile communication service. At this time, the signal from the mobile station is directly transmitted to the base station or is reflected by a certain building and then is transmitted to the base station. Namely, the signals transmitted to the base station may have a certain delay time based on multiple paths. The base station demodulates the signals from the mobile station received through multiple paths and transmits to a control station.
When the mobile station requests a call transmission, in a state that the mobile station does not receive a certain service from the base station, when transmitting a short signal to the base station, an access channel is used. The above-described access channel is a CDMA channel that the mobile station transmits a signal to the base station in the reverse direction in a state that a communication channel is not set between the mobile station and the base station. In addition, in this access channel, different offsets may allocated for preventing the signals from being concurrently transmitted from a plurality of mobile stations to the base station via the access channel.
FIG. 1 is a flow chart illustrating a conventional method for searching an access channel and allocating a finger for a base station in a conventional mobile communication system.
As shown therein, the method for searching an access channel and allocating a finger for a conventional mobile communication system includes steps ST1 through ST3 for searching a search energy for an access channel for each antenna, comparing with a search critical value and storing an offset which exceeds the search critical value, steps ST4 through ST6 for selecting an offset having the highest search energy among the stored offsets, allocating the selected offset to the finger for each antenna, and demodulating the same, and steps ST7 through ST9 for searching a multiple path signal of the offset for each antenna, allocating the offsets which are not allocated to the finger having a search energy which exceeds the search critical value to the remaining fingers for each antenna, and demodulating the same and then feeding back to the steps ST1 through ST3.
The operation of the method for searching an access channel and allocating a finger for a base station in a conventional mobile communication system will be explained with reference to the accompanying drawings.
First, in the base station for a mobile communication system, the space for smoothing a multiple path fading operation is diversed. Namely, a signal receiving state from the mobile station is improved using more than two receiving antenna. Here, two antennas are used, and a base station having four fingers is used. The finger performs a function for demodulating a signal from the mobile station to the base station.
In the step ST1, in the base station having two receiving antenna and four fingers, a search unit for the base station despreads a receiving signal at a certain offset interval for the receiving antenna and computes the search energy.
In the step ST2, the computed search energies which correspond to the same offset for each antenna are summed and compared with a set search critical value for judging whether the signal is detected from the mobile station.
In the step ST3, the search unit stores the offsets with respect to the signals having a search energy above the set search critical value.
In the steps ST4 and ST5, the offset having the highest search energy among the stored offsets is selected and allocated to the finger. At this time, the base station allocates to the finger-0 connected with the first antenna if the selected offset is a signal corresponding to the first antenna. If the selected offset is a signal corresponding to the second antenna, the offset is allocated to the finger_2 connected with the second antenna. The finger_0 and the finger_1 are connected with the first antenna, and the finger_2 and finger_3 are connected with the second antenna.
The finger_0 and finger_2 to which the selected offsets are allocated, compute the finger energy with respect to the signal of the allocated offset and compares with the set finger critical value. If the computed finger energy is above a set finger critical value, in the step ST6, the signal corresponding to the offset is demodulated.
At this time, in the steps ST7 and ST8, the search unit computes the search energy with respect to the offset for searching the multiple path signal for the antenna. The offsets having the highest search energy, not the offset allocated to the finger_0 and finger_2 among the offsets having the search energy above the search critical value, are allocated to the remaining fingers. At this time, if the signal of the offset having the highest search energy is a signal corresponding to the first antenna, the allocation is made for the finger_1, and if the offset signal is a signal corresponding to the second antenna, the allocation is made for the finger_3.
In the step ST9, the filger_1 and finger_3 having the allocated offsets demodulate the signals of the offsets.
In the step ST1, the search unit searches whether the signals having an energy above the search critical value provided to a new offset is received.
In the steps ST2 through ST4, if the signal having a search energy above the set search critical value is received as another offset, not the offset allocated to the finger_0 and finger_3, the offset of the signal having the highest search energy is selected. Thereafter, the new offset is allocated to the other finger which performs the demodulation function with respect to the offset as another offset is allocated. Namely, the offset of a newly searched signal is allocated to the finger_0 and finger_2.
The finger_0 and finger_1 demodulate the multiple signal having a different time offset received into the first antenna, and the finger_2 and finger_3 demodulate the multiple signal having a different time offset received into the second antenna. Namely, one finger is connected with one antenna for thereby demodulating one access channel signal.
The search unit repeatedly performs the allocation with respect to the finger by searching a new multiple path signal. The search unit of the conventional base station allocates an access channel signal from the other mobile station to the finger in order to protect the access channel signal from the other mobile station before the decoding operation of the access channel signal with respect to one mobile station is completed.
Generally, when using a communication channel between the base station and the mobile station for the mobile communication system, the signal is spread using a spreading code which is different based on each mobile station and is transmitted from the mobile station to the base station. In addition, in the base station, the communication channel signal from the mobile station is despread based on the same spreading code as the mobile station for thereby separating the signal from the other mobile station.
The signal with respect to the offset in which the search energy of the receiving signal which despreads using the spreading code with respect to a certain mobile station exceeds a search critical value is a multiple path signal that a certain mobile station transmits.
When using the access channel, the mobile stations spread the signals using the same spreading code and transmit to the base station, and the base station despreads the signals using the same spreading code and searches an access channel signal with respect to the mobile stations.
The signals received based on the different offsets may not be considered as an access channel signal of a certain mobile station but considered as a signal having the access channel signal of the other mobile stations.
The known base station allocates one finger for each antenna and demodulates in order to modulate and demodulate the access channel signal from a certain mobile station. Since the offset of other access channel signal is allocated to the finger before one access channel demodulation operation is completed, a certain interference may occur between the access channel signals of the other mobile stations. Namely, after the finger with respect to the initial signal is allocated, an interference that another offset is reallocated to the finger before the decoding operation with respect to the signal is completed may occur.
In addition, when searching an access channel multiple path signal, if there is a difference in the offset value, since the signal may not be an access channel signal of the current demodulation mobile station but be an access channel signal of the other mobile station, the operation for obtaining an access channel signal of the current demodulation mobile station may be failed during the process that the base station search unit continuously allocates the signal received based on the other offset to the finger when demodulating the access channel signal of a certain mobile station.
In addition, since the success ratio of the access channel signal decoding operation between the base station and the mobile station is decreased, the operation for transmitting an access channel signal from the mobile station to the base station must be re-performed, so that the load of the reverse direction link is increased.
Accordingly, it is an object of the present invention to provide a method for searching an access channel and allocating a finger for a base station which overcomes the problems encountered in the conventional art.
It is another object of the present invention to provide a method for searching an access channel and allocating a finger for a base station which is capable of preventing an interference for an access channel signal between the mobile stations by stopping a signal search operation of a search unit after a search unit of a base station allocates an offset to a finger with respect to an access channel signal of a certain mobile station and preventing a multiple path signal, which may be an access channel signal from a new mobile station during an access channel probe period which corresponds to a duration needed for obtaining one access channel signal by a base station, from being re-allocated to a finger.
To achieve the above object, there is provided a method for searching an access channel and allocating a finger for a base station which includes a first step for searching an access channel signal, selecting a highest search critical value among the offsets having a search energy above a search critical value, allocating a finger in accordance with a path via which the selected offset signal is received and demodulating the signal, and a second step for checking whether an access channel probe period which corresponds to the time required until an access channel message unit is received and decoded is elapsed, stopping a search of a new multiple signal component to minimize any interference of the deciding operation of an access channel signal of another mobile station when one access channel probe period is not elapsed and feeding back to the first step when the access channel probe period is elapsed wherein there is provided a method for searching an access signal from a base station to a mobile station and allocating a finger for a CDMA mobile communication base station.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.